Screwing tools with a device to limit transmitted torque are known in different systems, in application with hand screwdrivers and power tools. Of special interest under an aspect of the invention are such systems in which the torque is transmitted from the hand of the user through the handle of the tool to the shank of a screwdriver blade. Systems which have to be considered in relation to the system according to the invention are described in their construction and function below.
A screwdriver according to U.S. Pat. No. 5,746,298 with an adjustable torque limiting device is complicated in the construction of the torque transmitting coupling, which is made of metal. In the handle a rotatable shank is borne of which one end section is hexagon shaped in its cross-section. This end section is encompassed by six finger-shaped longitudinal end sections of a metal-sleeve which can turn relative to the shaft. Each of the finger-shaped end sections contacts one of the six flats of the six flats of the hexagon shaft-section and applies a spring force onto it. The finger-shaped end sections are encompassed by a collar-shaped element which is adjustable in a longitudinal direction and which applies a radial force onto the finger-shaped end sections, pressing them onto the flats of the hexagonal shaft section.
The closer the collar-shaped element is disposed to the end of the finger-shaped end section, the stronger the radial spring force applied onto the hexagonal shaft section and the higher the torque transmitted from the handle to the shank before the shank rotates relative to the sleeve with the finger-shaped end section via the handle, thereby interrupting the torque transmission. The production of this torque limiting device is expensive in its production. To adjust the desired torque-limit the handle has to be disassembled partly. This system therefore is not practical for use when different torque-values have to be adjusted.
DE OS 26 47 996 discloses a system which essentially consists of axial ball and roller bearings which are under axial load by a compression-spring. The balls and rollers are seated between discs and are disposed in cavities. One disc is connected with the torque-input element, and the other disc with the torque-output element of the device. If the torque exceeds a particular adjusted value, the force in circumferential direction will be higher than the axial force exerted by the spring holding the balls and rollers in their seats such that the balls and rollers come out of their seats and the discs rotate relative to each other, thereby interrupting the transmission of the torque.
DE OS 2427 352 describes a system in which the torque transmitting device consists of a first shank with a disc-shaped end section as a coupling element. Its front face has a wave-shaped profile in the circumferential direction. A second shank also has a disc-shaped end section as a coupling element with the same profile engaging with the end section of the first shank. By a compression of a pack of plate-ring-springs, an axial force is exerted on the first shank. If the torque is higher than the friction force between the two engaged coupling elements, they rotate relative to each other and the torque transmission is interrupted.
From DE OS 101 43 181 A1, a screwdriver with a device to limit a transmitted torque is known. The coupling between the handle and the screwdriver-shank consists of a tube-like metal part into which longitudinal short slots are cut. Small balls are sitting between disk-like coupling elements into which cavities are formed. The disk-like elements are under axial load via a compression spring. With a part of their bodies, the balls extend into the slots of the shaft tube. If the torque exceeds the adjusted value given by the pressure of the spring, the balls come out of their seats and are pressed into the slots in the shaft tube and the torque transmission is interrupted. The construction basically corresponds to the construction as described by DE OS 26 47 996. The device according to DE OS 101 43 181 A1 is expensive in its production because it consists of metal parts of high precision.
In a screwdriver with a torque-limiting device as described in EP 1 092 510 A2 a shaft is borne in a handle, said shaft having a spherical end surface to transmit an axial force onto the inner end surface of the hollow in the handle. The end section of the shaft is enlarged like a mushroom. The enlarged end section at its side opposite to its end surface is provided with a gear profile in the circumferential direction. This gear profile engages into a gear profile of corresponding shape which is formed into the rear end surface of a socket which is assembled axially moveable and rotateable onto the shaft. This socket is profiled at its circumferential surface with longitudinal flutes into which engage gear-like ribs standing radially from the surface of the hollow in the handle. Through these ribs, a torque is transmitted from the handle to the socket. A compression spring is assembled in the front section of the hollow and exerts an axial force onto the socket. The flanks of the teeth of the gear by which the socket and the enlarged end section of the shaft engage are inclined in one circumferential direction and are standing perpendicular to the longitudinal axis of the system in the other longitudinal direction. If a torque load is applied which is higher than the torque determined by adjusting a particular spring force in the direction of the inclined flanks, the teeth of the socket will slide at the flanks of the gear profile in the mushroom-shaped end of the shaft, lift out of the gear profile and rotate relative to each other. By this, the torque transmission from the handle to the shaft will be interrupted.
The disadvantage of this system is that the axial force of the spring is directed through the end surface of the shaft to the end surface of the hollow in the handle. This axial force is super-imposed by the axial force which is directed to the handle from the hand of a user. As a result, there will be relatively high friction between the end surface of the shaft and the end surface of the hollow. The torque value at which the transmission of torque will be interrupted is influenced not only by the axial force of the spring but also by the axial force of the hand respectively generating the friction between the end surface of the shaft and the end surface of the hollow in the handle. The desired torque limit will therefore not be kept within desired tolerances. Of advantage is that the essential parts of the device are produced of plastic material by injection molding. Of disadvantage is, however, that only a relatively low torque can be transmitted. At higher torque, plastic deformation of important parts occur, thereby reducing the functional precision of the system such that it may not be acceptable any more.
Some torque limiting devices as shortly described before are relatively expensive to produce because they are complicated in their construction and consist of metal parts formed by cutting, turning, milling, drilling etc. The adjustment of a particular torque can not be done easily or the functional precision is not satisfactory.
Nowadays, the demand for screwdrivers with a device allowing for easy adjustment of a limit for the transmitted torque has increased. Applications for such a tool include the fixing of tungsten-carbide cutting plates at machine tools and the assembling of electronic components or plastic parts. Corresponding with the increasing demand of this kind of tools is the demand for lower prices.
The object is to develop a tool of this kind which can be produced at relatively low cost, which can be used at torque loads covering the most applications for which such a tool can be used, which can be adjusted or set easily, which functions with high precision over a long time of use. This is solved by the subject of the invention.